The traditional concept of a dynamic balance between oxygen and glucose consumption to meet brain energy expenditures has been challenged by recent studies reporting that glucose utilization exceeds that for oxygen during and after mental testing and sensory stimulation. Others speculated that glycolysis is preferentially stimulated to support mental activity; increased channeling of glucose into non-oxidative pathways when the demand for energy is increased and when levels of oxygen in arterial blood are adequate is unexpected and is not understood. We are, therefore, elucidating metabolic requirements of functional activation to better understand the interactions between neurons and glia in working brain in vivo. We examined metabolism of [14C]glucose in rat brain when glucose consumption was increased and found that labeling of the brain lactate pool was increased 3-fold in K+-stimulated compared to resting tissue. Almost all of the excess glucose consumed in the activated tissue was converted to diffusible metabolites. About 20% of the glucose taken up by brain was lost to blood as lactate, and similar quantities must have been transported from activated tissue to other brain regions. Thus, trafficking of intermediary metabolites within the brain can be extensive; this is a new, unexpected finding. On the other hand, during and after sensory stimulation lactate accumulation and efflux to blood was much smaller even though the oxygen/glucose uptake ratio was reduced. We found that glycogen, which is localized mainly in glial cells, was depleted by sensory stimulation, indicating that replenishment of metabolic pools explains, in part, the excess consumption of glucose. Metabolism in both neurons and glia is altered when the brain responds to stimuli or carries out mental tasks and activities, and also during the recovery period following the activation period; the oxygen/glucose uptake ratio is influenced by both cell types.